It is recalled that admission and exhaust valves in internal combustion engines are opened and closed by a camshaft constrained to rotate with the drive shaft.
In order to open and close a valve at the chosen instant, it is essential for said valve to be held in contact with the corresponding cam on the camshaft.
That is why engines are equipped with return devices for each valve, each return device comprising a spring that urges said valve continuously towards its closed position (i.e. towards the corresponding cam).
Most of such return devices comprise mechanical springs which, when the engine speed is moderate, hold the valve continuously in abutment against the corresponding cam.
However, the main drawback of mechanical springs is that they start to resonate when engine speed becomes sufficiently high. That “valve hunting” phenomenon results in the movement in translation of the valve being dissociated from the movement in rotation of the camshaft.
As a result, considerable loss of power occurs.
Various solutions have been proposed for remedying that problem.
Thus, it is known that each valve can be equipped with a plurality of return springs of differing rates, in order to raise the resonant frequency of the resulting resilient system.
That solution is suitable for mass-produced engines whose operating speeds are quite moderate (i.e. their maximum speed generally does not exceed 8000 revolutions per minute (r.p.m.)).
However, that solution is too limited for motorbike and racing car engines whose maximum speeds are often in excess of 15,000 r.p.m.
Indeed, appearance of the valve hunting phenomenon has already been observed in that type of engine, even when the valves are equipped with multiple return spring devices.
In order to remedy that problem, in certain high-speed engines, it has been proposed to replace the mechanical springs with pneumatic springs, which are less likely to start resonating at high engine speeds.
Thus, a pneumatic return device for returning valves for internal combustion engines is known from Document FR-2 529 616, published some time ago.
That system includes a piston secured to a valve stem and slidably received in a cylinder forming a leaktight chamber that encloses a compressible fluid which is at a predetermined rated minimum pressure corresponding to the fully closed position of the valve.
Although that system has already given satisfaction, it does not make it possible to control the return force to which the valve is subjected.
Document U.S. Pat. No. 5,233,950 makes provision to equip the return device with means for regulating the pneumatic pressure prevailing in the cylinder in which the valve is slidably received.
Although the valve control system proposed in that document constitutes an improvement on the system of document FR-2 529 616, the implementation structure for pressure regulation is nevertheless relatively complex, and its insufficient reactivity proves to be detrimental when engine speed varies suddenly.